In wellbores, annular barriers are used for different purposes, such as for providing a barrier to flow between an inner and an outer tubular structure or an inner tubular structure and the inner wall of the borehole. The annular barriers are mounted as part of the well tubular structure. An annular barrier has an inner wall surrounded by an annular expandable sleeve. The expandable sleeve is typically made of an elastomeric material, but may also be made of metal. The sleeve is fastened at its ends to the inner wall of the annular barrier.
In order to seal off a zone between an inner and an outer tubular structure or a well tubular structure and the borehole, a second annular barrier is used. The first annular barrier is expanded at one side of the zone to be sealed off and the second annular barrier is expanded at the other side of that zone. Thus, the zone is sealed off.
An annular barrier having an expandable metal sleeve is known from U.S. Pat. No. 6,640,893 B1. In its unexpanded condition, the inner wall of the annular barrier and the enclosing expandable sleeve form a chamber. When the annular barrier is installed forming part of the well tubular structure string, the chamber is prefilled with hardening cement through openings in the inner wall of the annular barrier. This is performed in order to prevent fluid flowing inside the well tubular structure during production from entering the openings and thus the chamber.
The sleeve is expanded by injecting a second cement compound into the chamber through the openings and thus expanding the sleeve by breaking the hardened cement. If the chamber had been filled with fluid and not hardening cement, the second cement compound would be diluted and thus be unable to harden subsequently. In order to provide the second cement compound with sufficient pressure, the well tubular structure is closed off at the end most distant from the surface and the well tubular structure is filled with the second cement compound.
When mounting the well tubular structure string, annular barriers can be inserted at regular intervals. Some annular barriers may be used to fasten or centralise the well tubular structure string in the borehole, whereas others await a later use, such as sealing off a zone. Cement prefilled in the chambers may thus have to wait for expansion at the risk of losing its properties before use.
When the annular barriers of U.S. Pat. No. 6,640,893 B1 are used for centralising or sealing off a production zone, the second cement compound filling up the well tubular structure and, subsequently, also the plug have to be removed. This is a costly procedure requiring several steps subsequent to the step of expanding the sleeve.
Furthermore, the first cement compound may close the opening so that the opening has to be cleaned before injecting the second cement compound. The opening may also be filled with contaminants or fragments comprised in the fluid running in the well tubular structure during production.